This invention relates to a cross-polarization interference canceller for use in a digital radio communication system which transmits digital modulated signals over ratio frequencies having planes of polarization orthogonal to each other.
Digital radio communication systems transmit digital modulated signals, such as multi-level quadrature amplitude-modulated signals or multi-phase phase-modulated signals, over a mutually orthogonal pair of a vertical polarized wave and a horizontal polarized wave. In such systems, as long as the orthogonality between the vertical polarized wave and the horizontal polarized wave is maintained, signals transmitted over either one of the orthogonally crossing polarized waves do not interfere with the other. They may interfere with the other, however, if this orthogonality is disturbed by the effect of fading, rain or the like on the transmission path. This interference is known as cross-polarization interference. For the elimination of this cross-polarization interference is used a cross-polarization interference canceller.
For this kind of cross-polarization interference canceller, reference may be made to a paper entitled "CROSS-POLARIZATION INTERFERENCE CANCELLATION IN THE PRESENCE OF DELLY EFFECTS" by B. Lankl, J. A. Nossek and G. Sebald, in Conference Record of IEEE International Conference on Communications '88 held in Philadelphia, on June 12-15, 1988, Volume 3 of 3, pages 1355-1361. Toru Matsuura on July 27, 1990 applied for a U.S. patent on "Cross-Polarization Interference Cancellation System Capable of Stably Carrying Out Operation" under application Ser. No. 07/558,754, and on the same day also filed an application with E.P.C. for a patent on "Cross-Polarization Interference Cancellation System Capable of Stably Carrying Out Operation" under Application No. 900114460.0.
The transversal filter-based cross-polarization interference canceller in these systems, however, carries out high speed operation at double the frequency of the symbol rate so that it may not lose its cross-polarization interference cancelling capability even if a lag occurs between the transmission delay time of the main signals on one polarized wave and interfering signals on the other. Therefore, if this cross-polarization interference canceller is composed as a fully digital processing type, the digital multiplier and the digital adder in the circuit will have to be operated at high speed. Such a digital multiplier usually is expensive and consumes much power.
Meanwhile, even if the cross-polarization interference canceller is large-scale integrated (LSI) by using complementary metal oxide semiconductors (CMOS's), they have to be able to operate at a high frequency, and such CMOS's also consume much power.
Conceivably, the processing speed can be increased by connecting more than one low-speed cross-polarization interference canceller in parallel, but this arrangement would require a plurality of cross-polarization interference cancellers and a circuit for controlling their parallel operation, which again means the disadvantages of complexity, a high cost and high power consumption.